The present invention relates to forming and crystallizing low molecular weight polymer particles. More particularly, this invention relates to a method and apparatus for extruding, cutting and crystallizing polymer particles in a liquid medium.
Polymer resins are molded into a variety of useful products. Useful polymer resins include aromatic polyesters of which polyethylene terephthalate (PET), polytrimethylene terephthalate (PTT), polybutylene terephthalate (PBT), polytrimethylene naphthalate (PTN), polycyclohexyl terephthalate (PCT) and polyethylene naphthalate (PEN) are examples. Polyester resins, particularly PET, copolymers of terephthalic acid with lower proportions of isophthalic acid and PBT are used in the production of beverage containers, films, fibers, packages and tire cord.
Polyester resin is produced in a melt phase polymerization (MPP) process with relatively low molecular weight inadequate for commercial uses. The molecular weight of MPP product must be upgraded. Consequently, the MPP product is formed into particles and subjected to solid state polycondensation (SSP) to increase its molecular weight by maintaining the solid polymer particles at temperatures between the glass transition and the melting point temperatures while removing the reaction products under an inert gas sweep or vacuum.
Molten polyester resin from the MPP is typically extruded under pressure and cut into small particles. U.S. Pat. No. 4,436,782 discloses a machine for forming molten pellets of PET and quenching the pellets in water. GB 1,143,182 teaches a die-face polymer cutter with the die face submerged in water to immediately quench pellets upon forming. WO 00/23497 A1 discloses extruding the molten polymer into strands, quenching the strands in cooling liquid and cutting the strands into polymer particles.
According to U.S. Pat. No. 4,064,112, the tendency of the particles to agglomerate due to stickiness during solid state polycondensation (SSP) can be reduced and even eliminated if the solid state polymerization is preceded by a crystallization step which comprises thermal treatment. A process described in U.S. Pat. No. 5,540,868 forms low molecular weight polyester particles with a degree of crystallinity greater than about 15% suitable for use as an SSP feedstock. U.S. Pat. No. 5,290,913 discloses crystallizing PET particles in an agitated liquid bath and heating to crystallization temperature. U.S. Pat. No. 5,532,335 and WO 00/23497 A1 teach crystallizing polyesters in liquid over 100° C. Processes disclosed in U.S. Pat. No. 6,740,733 B2, U.S. Pat. No. 6,297,315 B1 and U.S. Pat. No. 6,461,575 B1 separate relatively cool water used in pelletizing from PTT pellets and crystallize the pellets in relatively warm water at no more than 100° C. WO 00/23497 A1 discloses cooling PET during or after forming and then crystallizing PET pellets at above 100° C.
The process in WO 2004/033174 A1 entails granulating polymer in a liquid bath or immediately conducting granulate into a liquid bath with a temperature above 100° C. Following crystallization, the granulate-liquid mixture is cooled down to around 60° C., admixed with a cooler liquid, and depressurized after which the granulate is separated from liquid.
U.S. Pat. No. 6,749,821 shows that in a typical SSP process, polymer particles are delivered to an SSP reactor system through a hopper to a heated, fluidized bed pre-crystallizer operating to achieve a degree of crystallinity. The polymer particles are then fed into a first crystallizer and then optionally into a second crystallizer. The crystallizers heat the polymer particles under mechanical agitation to bring them to the desired reaction temperature and degree of crystallinity suitable for the ensuing SSP reactor. Polyester polymers undergo exothermic heat of crystallization if not crystallized to a sufficient degree. The continuance of the crystallization process in the SSP reactor leads to problems of heat release and agglomerization or sintering of the particles, causing maldistribution of gases and solids flow interruptions. The inlet of the tall SSP reactor is high above the ground, so the particles will have to be lifted to the inlet to enter the SSP process. In industrial practice, this is usually by slow motion pneumatic conveying.
The melting point, Tm, of a polymer is preferably determined as the maximum of the melting endotherm on the first heat, measured by Differential Scanning Calorimetry (DSC). Glass transition temperature (Tg), is the inflection point of the step transition associated with the glass transition on a DSC trace heated at about 10° C./min. Average bulk temperature of a polymer particle is the average temperature of the mass of the particle or the average of the temperature in every location of the particle. The term “measured or actual maximum crystallization rate temperature (Tc)” is the experimentally determined definition known in the art. The determination of Tg and Tm is described in ASTM D-3419-99 “Standard Test Method for Transition Temperatures of Polymers by Differential Scanning Calorimetry”.
The temperature of maximum crystallization rate (Tc) can be experimentally determined by several methods. One such method is to observe the initial growth rate of spherulitic structures from a thin slice of amorphous melt by polarized-light microscopy, make photographic measurements and plot the rate versus the applied temperature. This and other methods are described in J. M. Schultz, “Polymer Crystallization: The Development of Crystalline Order in Thermoplastic Polymers”, 2001 Oxford U. Press 127-139.
Values of Tc may be found in the literature for a wide range of polymers. According to U.S. Pat. No. 5,540,868, it is also possible to estimate Tc calculating the average of the glass transition and melting temperatures using Formula (1):Tc=(Tg+Tm)/2  (1)
For the purposes of this invention, this value of Tc is a reasonable approximation of the true temperature of maximum crystallization rate. U.S. Pat. No. 6,451,966 B1 gives approximate Tg and Tm values for some useful polyesters to which is added calculated Tc values in the table below.
CalculatedTg, ° C.Tm, ° C.Tc, ° C.PET70260165PEN120270195PBN82242162PTT35227131The values for Tg, Tc, and Tm can vary somewhat, for example, with the morphology, thermal history, molecular weight, moisture level and initial crystallinity on a polymer. For example, PET with a low degree of polymerization between 10 and 20 typically has a Tm of about 250° C., a Tg of about 60° C., and a Tc of about 155° C. U.S. Pat. No. 5,744,578 teaches crystallizing PEN in liquid at a temperature in the range of about 20° C. above a glass transition temperature (Tg) and about 10° C. below a melting temperature (Tm).
An object of the present invention is to quench freshly formed molten polymer particles in a cooling liquid to solidify the exterior of the polymer particles while still maintaining average bulk temperature of the polymer particles in the range at which they crystallize at a reasonable rate.
Another object of the present invention is to replace the aforementioned cooling liquid containing the solid polymer particles with a warming liquid that has a temperature at or above the aforementioned average bulk temperature of the polymer.
A further object of the present invention is to upwardly transport solid polymer particles in liquid to the SSP reactor system.
A further object of the present invention is to crystallize solid polymers in liquid while transporting the polymer in liquid to an SSP reactor system.